1. Field of the Invention
The present invention relates to a projection exposure apparatus for forming fine patterns required for manufacture of LSIs.
2. Description of the Related Art
FIG. 7 shows an optical system of a conventional projection exposure apparatus. The optical system includes a lamp house 1, a mirror 2, a fly's eye lens 3 disposed in front of the lamp house 1 with the mirror 2 between the lamp house 1 and the fly's eye lens 3, an aperture member 4 located in front of the fly's eye lens 3, condenser lenses 5 and 6, a mirror 7, a photo mask 8 including a circuit pattern, a projection lens 9, and a wafer 10 located in front of the mask 8 with the projection lens 9 between the mask 8 and the wafer 10. The aperture member 4 has a disk-like shape including a circular open portion 4a , as shown in FIGS. 8 and 9.
Light rays emanating from the lamp house 1 reach the fly's eye lens 3 through the mirror 2, and are split into regions by the individual lenses 3a of the fly's eye lens 3. The rays which have passed through the individual lenses 3a pass through the aperture portion 4a of the aperture member 4, the condenser lens 5, the mirror 7 and the condenser lens 6, and then illuminate the entire surface of the exposure area of the mask 8. Therefore, on the surface of the mask 8, the rays from the individual lenses 3a of the fly's eye lens 3 are laid on top of one another, and the mask 8 is thus uniformly illuminated. The light rays which have passed through the mask 8 in the manner described above pass through the projection lens 9 and then reach the wafer 10, whereby transfer of the circuit pattern on the surface of the wafer 10 is achieved.
In such a projection exposure apparatus, the minimum resolution R is proportional to .lambda./NA, where .lambda. is the wavelength of the illumination light and NA is the numerical aperture. Thus, the resolution of the conventional projection exposure apparatus is improved by designing an optical system such that the numerical aperture NA is increased, thereby providing an exposure apparatus which can cope with the increase in the degree of integration of LSIs has taken place in recent years.
However, although an increase in numerical aperture NA of the optical system improves the resolution R, it further reduces the depth of focus DOF of the projection exposure apparatus. The depth of focus DOF is proportional to .lambda./NA.sup.2. Therefore, in a conventional projection exposure apparatus, an increase in the resolution reduces the depth of focus and thus deteriorates the transfer accuracy.
Here, the numerical aperture NA of the optical system is expressed by sin .theta., where .theta. is the angle of incidence of the light incident on a wafer 10. Thus, the larger the angle of incidence .theta. with respect to the wafer 10, the shorter the depth of focus DOF.
Hence, Japanese Patent Laid-Open No. 61-91662 has proposed a projection exposure apparatus which employs a ring-shaped aperture member. In this exposure apparatus, only the light which has passed through the peripheral portion of the aperture member is utilized by employing the ring-shaped aperture member. Consequently, in the diffracted light of the first order which is incident on the wafer, the components having a large angle of incidence are blocked, whereby the numerical aperture NA of the optical system is reduced and hence the depth of focus DOF is improved. Therefore, it has been described in the above-mentioned invention that the resolution is improved while the depth of focus DOF is enlarged by employing only the light which passes through the outermost portion of the aperture member.
However, when a large ring-shaped light source is employed so that only the light which passes through the outermost portion of the aperture member is used, as described in the above-mentioned publication, a light source image S.sub.0 of the zeroth order and light source images S.sub.1 and S.sub.2 of .+-. first order are produced on the surface of a pupil 9a of the projection lens, as shown in FIG. 10. As a result, the angle of incidence of the light of the zeroth order is increased, and hence the depth of focus DOF deteriorates. In FIG. 10, hatched portions P.sub.0 through P.sub.2 indicate the light blocked portions formed by the ring-shaped aperture member.